Maintenance DNA methylation in mammalian genome is required for transcriptional gene silencing, X chromosome inactivation and heterochromatin formation. The mammalian maintenance DNA (cytosine-5) methyltransferase 1 (DNMT1) contains several domains, including a domain targeting replication foci, a DNA-binding “CXXC” domain, a pair of “bromo-adjacent homology” or “BAH” domains, and a C-terminal catalytic domain that shares homology with most bacterial cytosine-5 methyltransferases. DNMT1 plays a significant role during development, and is required for embryonic survival in mice. DNMT1 adds methyl groups to hemimethylated DNA during DNA replication. Other DNA methyltransferases in mammals include DNMT-3A and 3B.
In human cancer, the DNA methylation pattern becomes aberrant, resulting in hypermethylation and transcriptional silencing of the promoters of a number of tumor suppressor genes. Restoring the expression of hypermethylated tumor suppressor genes by inhibiting the DNA methyltransferases (DNMT1, DNMT3A, and DNMT3B) has emerged as a desirable strategy against cancer. DNMT1 is more highly expressed when cells are replicating their DNA, and is therefore a promising target for inhibiting methylation in rapidly dividing cells such as cancer cells. The DNA methyltransferase (DNMT) inhibitors azacytidine and decitabine are the most successful epigenetic drugs to date for clinical use in mixed-lineage leukemia (“MLL”), myelodysplastic syndrome (“MDS”) and acute myelogenous leukemia (“AML”) patients, and are still the most widely used as epigenetic modulators, even though their application is restricted by their relative toxicity and poor chemical stability in vivo. Zebularine (1-(β-D-ribofuranosyl)-1,2-dihydropyrimidin-2-one), a more stable cytidine analog, is another inhibitor of DNMT with concomitant inhibitory activity towards cytidine deaminase. Although many new inhibitors of DNMT have been identified, none of them can so far replace azacytidine, decitabine and, to a lesser degree, zebularine.